* MIUI camera gives F.C. You can fix that if enter in setting menu of camera app, Picture size and choose lower resolution, like 2048×1536 and can be desabled Auto focus option, is useless.
* Launcher and Setting menu is laggy, what you can do is to try other Launcher or, at least, disable this options:
-on Laucher „Hi-res graphics” and „Icon shadow”.
-on Settings menu turn off system „Animation”, screen on&off anim
*Apply only lighter themes for system and don’t use Live wallpaper.
*Stuck on HDPI lockscreen just touch „back” button and press „volume +” key and release.
*MDPI LOCKSCREENS : http://forum.xda-developers.com/showthread.php?t=1428940

I don’t think that this tips makes MIUI to fill much more fluid but helps. There is no official support for mdpi screen, only for higher resolution phones and with powerfull hardware😦 .